1. Technical Field
Embodiments of the present disclosure relate generally to image sensing systems, and more specifically to techniques for mitigating the effects of signal overload in analog front-end circuits used in image sensing systems.
2. Related Art
Image sensing systems generally refer to systems used for capturing images of objects. The captured images may be suitably processed for desired applications and/or displayed. An example of an image sensing system is a digital X-ray machine. Image sensing systems typically use image sensor arrays. Sensors in the image sensor arrays generate a charge proportional to the intensity of energy or rays (such as visible light, X-rays, infra red light, etc.) incident on the sensors (and thus representative of the image to be captured). The term ‘energy’ is used generically herein to include rays such as those noted above. Each sensor typically corresponds to one “pixel” of the image to be captured.
Sensor arrays of an image sensing system may be connected to analog front-end (AFE) circuits. AFE circuits are typically used for amplification of the signals generated by the sensors, conversion of the signals from charge to voltage and digital representation of the voltage, etc. For example, in a digital X-ray machine, the AFE may include charge-to-voltage converters, a multiplexer and one or more analog to digital converters (ADC). The charge-to-voltage converter is typically implemented by an amplifier configured as a signal integrator. One problem that may occur in an AFE (and more specifically in the charge-to-voltage converter in the AFE) of an image sensing system is signal overload. Signal overload of a charge-to-voltage amplifier occurs when the magnitude of charge received as input is such as to cause the output voltage of the charge-to-voltage amplifier to exceed desired operating thresholds. Such signal overload may be caused by a large amount of energy being incident on a pixel, resulting in an excessively ‘bright’ pixel. Other causes of signal overload may include readout of a defective pixel/sensor in the image sensor array. Signal overload may result in images generated by the image sensing system to be blurred or otherwise not faithfully representative of the imaged object.
One prior technique to address the problem noted above employs an additional transistor in each pixel (sensor) in the image sensor array, the transistor operating as a clamp to prevent signal overload of the charge-to-voltage amplifier. Another prior technique reduces the possibility of such signal overload by laser-cutting defective sensors in the sensor array. A yet another prior technique employs special materials with smaller dielectric constant and adsorption for implementing the signal lines (interconnect) from each sensor.
The prior techniques noted above are implemented in the image sensor arrays (and not in the AFE), and may be associated with increased cost. Further, having to rely on such implementation in the image sensor array provides lesser flexibility for a system integrator in integrating sub-systems, components and modules that make up an image sensing system.